In the cooking or heating of foods in a microwave oven, it is sometimes desirable to provide a supplemental heat source. Providing a hot surface adjacent to the food surface performs a number of useful purposes, among them being enhancing the crisping of a microwave heated food surface which would otherwise be soft or soggy, browning a food surface to provide desirable color and accelerating the heating of low loss foods such as oil in microwave popcorn, resulting in better product performance.
Heaters able to achieve these effects can take various forms. In general these heaters (susceptors) convert microwave energy into thermal energy.
Such susceptors may be described as devices positioned in close proximity (heat transfer relationship) to a food surface and are capable of preferentially absorbing microwave energy such that the food surface heats faster or hotter or both than it would when exposed to microwave energy without a susceptor present.
Such heaters can be in the form of a disposable packaging component or a utensil. Incorporating a susceptor into a disposable package provides a very desirable increase in user convenience and satisfaction.
Typical of disposable package heaters are the metallized polyester heaters as disclosed by Seiferth, U.S. Pat. No. 4,641,005 and Brastad, U.S. Pat. No. 4,267,420.
The heaters disclosed by Seiferth and Brastad are metallized polyester. The layer of metal must be thin in order to work. The thickness of the metal layer must be on the order of less than about 3-5.times.10.sup.-6 cm (300 to 500 angstroms) or the metal will not heat enough to cook food. In practical terms, thick metal layers act as reflectors and prevent the transmission of microwave energy therethrough to a food product. That is, its absorption and transmission are, for practical purposes, zero.
European patent application Ser. No. 0,242,952 discloses a composite material for use as a microwave heater. The material comprises a dielectric substrate coated with a mixture of an electrically conductive metal or metal alloy in flake form in a thermoplastic dielectric matrix. The DC surface resistance of the resulting composite is greater than 1.times.10.sup.6 ohms per square, thereby eliminating resistance characteristics as the heating mechanism. Further, the flakes used in the material are relatively thick and on their own would not be lossy at the thickness of 0.1 to 0.5 micrometers (microns). Further it is disclosed that the flakes are substantially insulated from each other, precluding electrical conductivity from one portion of the heater to the next. In addition, the flakes are required to have a high aspect ratio, that is, in the range of 10 to 300. The aspect ratio is defined as the ratio of the largest dimension of the face to the thickness of the flakes.
U.S. Pat. No. 4,518,651 (Wolfe) discloses a microwave heater which uses a lossy layer which is relatively thin. The heater is manufactured by coating a substrate with a liquid. The heating layer is pressure formed into the substrate to provide heating. After pressure forming the thickness of the layer is less than 10 micrometers. It can be seen from the disclosure at column 8 that an increase in thickness achieved by increasing the coating weight does not increase heating.
The metal flakes enrobed in a dielectric substance disclosed by Wolfe uses a complex system for production. Also, Wolfe's susceptor films are quite thin, being on the order of 12 microns (0.0005") thick.
Layers of semiconducting material can be adhered to metal substrates in which case the lossy material can be magnetic in loss characteristics. See, for example, Anderson, U.S. Pat. No. 4,283,427.
The above referenced heaters can take one of several forms, for example a continuous thin-film coating, islands of thin-film coating, pellets or thick layers adhered to a substrate.
A number of containers have been proposed for browning or searing the surface of the food and fall into three general categories. The first are those which include an electrically resistive film, for example tin oxide, usually less than about 1.times.10.sup.-5 cm to 2.times.10.sup.-5 cm thick that is applied to the surface of a nonconductor such as a ceramic dish and described, for example, in U.S. Pat. Nos. 3,853,612; 3,705,054; 3,922,452 and 3,783,220. Heat is produced because of the I.sup.2 R loss (resistive loss). This system is not acceptable for use in the invention primarily because of the bulk, weight and cost of the dish and its breakability. Ceramic dishes, with lossy coatings used in a microwave oven are relatively expensive. A second type includes a microwave energy absorber formed from a mass of particles that become hot in bulk when exposed to microwave energy. The microwave absorbing substance can be composed of ferrites, carbon particles, etc. Examples of these are described in U.S. Pat. Nos. 2,582,174; 2,830,162; 3,302,632; 3,773,669; 3,777,099; 3,881,027; 3,701,872 and 3,731,037 and German Pat. No. 1,049,019.
The third category is exemplified by the currently used commercially available disposable heater uses a polyester layer with a very thin metal layer thereon. Such heaters substantially change heating characteristics during operation, which can be a detriment in some food preparation situations. Another problem, for example, with the metallized heaters, is that they tend to change with time perhaps through an oxidation process and therefore their performance characteristics can vary which can also result in different cooking results.
The metal coated polyester films as disclosed by Seiferth and Brastad are generally incapable of sustained, consistent suscepting in that the metal coated polyester surfaces break up under ordinary microwave heating or cooking conditions at which point suscepting by the metal coated film (i.e. preferential microwave power absorption) is much reduced or ceases.
The above-described first two types of supplemental heaters, as discussed, generally exhibit one or more problems in their use or manufacture particularly when the heater is meant to be disposable.
The ceramic dishes with tin-oxide coating are too expensive, too bulky, require warm-up time, are breakable and retain heat for a long time so as to be inconvenient to handle and use.
The second type of heaters is too bulky or expensive for practical use as disposable heaters, i.e. one which is used once and then disposed of.
The above types of heaters can be difficult to make and can be expensive to manufacture and can require sophisticated manufacturing equipment. Further, there can be limits on the types of materials used as lossy substances and the substrates onto which lossy material is applied e.g. commercially available metallized heaters use very smooth surfaced substrates like polyester.
A major drawback of these heaters is that they can produce a significant contrast or nonuniformity in temperature across the surface, providing uneven cooking results. A solution to the nonuniformity problem, amongst other things, can be found in U.S. patent application Ser. No. 119,381 to Dan J. Wendt et al, the disclosure of which is incorporated herein by reference. The Wendt invention has provided, amongst other things, an effective means to improve temperature uniformity of heaters by using an additional element.
The art also discloses non thin film heaters, for example that disclosed by U.S. Pat. No. 4,190,757 to Turpin which uses a relatively thick lossy layer. Thick heaters can be costly and difficult to manufacture.
The Turpin, et al. device must be thick (0.016" to 0.125") to be effective. The resulting structure, while very effective in heating foods needing crisping such as pizza, is massive, slow to heat, and too expensive to be acceptable as a disposable food package. It also used microwave shielding for the food so that during the time needed to get the susceptor materials up to operating temperature to brown or crisp the food, the food was not overcooked.
Ideally, a susceptor should be capable of very rapidly reaching and then maintaining a suitable susceptor and food surface temperature without having to separately preheat the susceptor as commercially available microwave browning dishes require.
Different foods have different suscepting needs--for example, popcorn vs. pizza. Also, the same type of food may have different suscepting needs due to its shape and/or differences in its thickness. Also, it may be desirable to absorb less power at the edges of a susceptor than toward its center in order to get good heating in the center without overheating at the edge.
An effective and economical means of controllably varying susceptor performance is clearly of great value.
By the selection of electrical and/or magnetic loss properties for the lossy layer as hereinafter taught, power penetration depths can be achieved in the lossy layer which permit intermediate thickness susceptor layers which will produce a range of heat to heat a wide variety of foods in a microwave oven, achieving the aforedescribed suscepting function. The invention also provides means for producing heaters with a predetermined temperature profile across the surface in an easy and effective manner and allows production of heaters with processes and machinery that provide both speed and economy and which are readily commercially available, reducing the need for designing and building complicated or specialty processing lines. Further, within the invention range of thicknesses, variation of power absorption across the lossy layer thickness due to interference effects as a result of reflections within the microwave absorbing layers, is minimal.
It has been found that by the proper selection of materials, their electric and magnetic properties (defined together in the term power penetration depth), and appropriate thickness, a heater can be provided which overcomes many of the above problems. When the lossy material is applied in liquid form during manufacture, a thin layer is provided which after solidifying (curing) has sufficient lossy characteristics to heat food. After application of the lossy material to a suitable substrate, the materials cure, i.e. a portion of the vehicle either evaporates or changes form to provide a nonliquid coating. The coating can be applied in a continuous film, in the form of a grid or in the form of an array of discrete discontinous areas separated from one another or in patterns of preselected thickness and/or materials or combinations thereof.
The substrate can be any material which has sufficient thermal stability, as it relates to, for example, objectionable discoloration, odor, degradation, etc., to withstand the operating temperature of the heater. This includes both substrates that are relatively microwave transparent such as paper, paperboard, plastics and other polymers and also substrates that are microwave reflective such as aluminum foil.
An object of the present invention is to provide a microwave heater which is easy to manufacture by one or more of several liquid coating techniques. Another object of the present invention is to provide a heater which can be made by a process which allows forming a predetermined patterned lossy layer to provide either uniformity of heating or a predetermined heating distribution, something which current commerically available heaters have not been able to achieve. A still further advantage of the present invention is to provide a heater and a heater making process which allows easy changing of the performance characteristics by changing separately or in combination either the proportions of the coating lossy material(s), the type(s) of lossy material, the thickness of the lossy layer or the vehicle. A further object of the present invention is to provide a lossy layer which can be applied to a broader range of substrates than other forms of microwave heater lossy layers. Another object of the present invention is to provide a heater and method of manufacture which allows a wide range of operating characteristics including heaters which are substantially interactive with the microwave by electrical field induced heating or with magnetic field induced heating or combinations thereof. Another object and advantage of the present invention is that heater performance characteristics can be varied for greater latitude in heater performance characteristics desired for optimum food preparation or reconstitution. A still further object of the present invention is to provide a microwave heater which is stable as it relates to dimensions and/or to operating characteristics during heating. Another object of the present invention is to provide a microwave heater which has lossy layer thickness that provides ease of manufacturing. Another object of the present invention is to provide a microwave heater which can utilize a broader range of materials for substrate material and lossy layer material than some of the currently available heaters.